Main types of resonators that are used in high frequency bands and microwave bands are uniform line resonators of quarter-wave type or half-wave type. In these days, stepped impedance resonators (hereinafter also referred to as SIRs) comprised of a plurality of transmission lines of different line impedances as shown in Non-Patent Document 1 are being increasingly used for the purpose of realizing downsizing, spurious suppression or various coupling methods.
There are various configurations for the SIRs. Representative ones are one-end open and other-end short-circuited type SIRs of quarter-wave type and both-end open type SIRs of half-wave type. As shown in the Non-Patent Documents 2 and 3, since downsizing can be best achieved when using quarter-wave type SIRs, active developments and implementations have been made thereof for a long time. In these years, thanks to establishment of process technologies of LTCC (Low Temperature Co-fired Ceramics), quarter-wave SIRs are now often used in microwave band radio system filters as shown in Patent Document 1.
Both-end open type SIRs of half-wave type can be easily realized by using strip lines or microstrip lines, and are practically offered for application as small-sized hairpin resonators or split ring resonators configured in U-shape or rectangular loop-like shape.
However, it has been indicated that downsizing and reduction of loss (or higher Q values of the resonator) do not coexist in both-end open type SIRs of half-wave type. While a both-end short-circuited type SIR might be an option, it will result in a drawback that the resonator increases in size.